The present invention relates to a bias current recovery circuit, in particular for saw-tooth wave generators.
As is known, saw-tooth wave generators are circuits which generally comprise a current source stage (usually a current mirror) which is controlled from the outside by a settable value current source and which feeds a capacitor also connected to a control element, typically a transistor, which is switched on by short pulses (see for example figure 1). Accordingly, during the phase in which the transistor is OFF, the current mirror circuit feeds the set current to the capacitor which is thus charged in a linear manner, while during the short intervals with the control transistor in the ON state discharge of the capacitor occurs through said transistor. With such a circuit, the operating frequency is in practice controlled by the equation EQU I t=C.DELTA.V
wherein
I is the capacitor loading current through the current source stage, C is the capacitance of the capacitor, PA1 .DELTA.V is the voltage variation across the capacitor, while t is the charge time which in practice coincides with the period T since discharge is much faster than charge.
Moreover, as is known, in saw-tooth wave generator circuits for the Y-axis used in data displays, the performance as to precision and temperature stability must be better with respect to circuits of other types, in particular with respect to standard television circuits. In particular, for designing a saw-tooth wave generator with a period T comprised between 10 and 20 ms, using a capacitor C=100-200 nF it is necessary to charge this capacitor with a current having a rather small value, and then discharge it with a relatively high current so as to achieve the required saw-tooth shape. In data display circuits, the value of the capacitor which appears in the above equation is determined by the requirements of good temperature stability, low tolerance on the nominal value and naturally not too high cost, which are fundamental for the application being considered. Accordingly, to obtain a precise operating frequency it is necessary for the current I supplied to the capacitor through the current source circuit to be exactly equal to the one which loads the capacitor in a linear manner. On the subject it should be stressed that, due to the values of the variables involved in the above mentioned equation, the current I is comprised between 10 and 100 .mu.A, and is therefore rather small and as such is particularly responsive to external influences which can thus vary the operating frequency of the circuit in its entirety.
In fact, the high impedance signal present across the capacitor is fed to the downstream user stages by means of a separator circuit or buffer with a low-impedance output, so that these stages can use it and process it as required. However, the buffer stage (also illustrated in FIG. 1) introduces an error in the charge current fed to the capacitor due to its bias current. Typically, the error caused by the buffer stage is due to the base current of a PNP or NPN transistor which is added to or subtracted from the current supplied by the source stage and, due to the low value of the currents, considerably affects the charging slope of the capacitor and thus, finally, the frequency of the signal obtained.
The problem is furthermore increased by the fact that said base current does not remain constant as the temperature increases, but varies with it due to the temperature dependence of the current gain h.sub.fe, which generates a frequency drift.
To solve the problem of the error introduced by the bias current of the buffer stage on the operating frequency of the generator circuit, attempts have been made, for example, to design the buffer stage as a pair of transistors connected to one another in the Darlington configuration, thereby reducing the influence of the base current on the charge capacitor. However, this solution has not proved satisfactory due to the residual error introduced by the base current of the Darlington and most of all to the fact that in any case this current is still responsive to temperature variations, which case a frequency variation according to the operating temperature of the circuit.